1. Field of the Invention
This invention relates to fiber lasers, and particularly to methods and means for coupling radiation into the core of a fiber laser oscillator or optical fiber laser amplifier.
2. Description of the Prior Art
Fiber lasers are useful in printers to form images by exposing a medium while scanning it. Such media may, for example, be of the thermally sensitive type that exhibits a binary response, one requiring that a threshold level of energy be exceeded for image formation purposes. Such fiber lasers are often pumped by radiant sources which deliver radiation to the active material in the fiber's core, which serves as the lasing cavity. For optimal performance in such devices, it is desirable to make the most efficient use of available pump power.
U.S. Pat. No. 4,815,079 discloses an optical fiber laser in which radiation is pumped into a single mode core of a laser cavity via a double clad fiber coupling arrangement. Here, a first multimode layer surrounds a single mode core, and it, in turn, is surrounded by a second outermost layer. The multimode layer is a core with respect to the outermost layer which serves as a cladding. At the same time, the multimode layer is a cladding with respect to the single mode, innermost, core. Pump light coupled into the first multimode cladding, either through its end facet or its side, propagates along its length, undergoing a multiplicity of reflections at its core cladding interface, while intersecting and being absorbed by the innermost core to cause lasing action and/or amplification. In one type of fiber laser the multimode waveguide or cladding takes the form a rectangular slab that extends along the laser length.
Fiber lasers are usually made fairly long, on the order of thirty meters or so, with small cross-sectional geometries to make them easy to coil into compact configurations and prevent concentration quenching. However, these qualities, while advantageous for a variety of reasons, make it extremely difficult to efficiently couple high pump power into them to promote lasing action, particularly where printing applications demand high power. Those skilled in the art have addressed the foregoing problem with more or less success in a variety of ways.
For example, U.S. Pat. No. 4,818,062 discloses a fiber laser with a bundle of waveguides that receive light from a bar of diode laser elements and couple their outputs into a solid state laser medium through a lens. While such an arrangement is adequate for solid state lasers having entrance facets with dimensions of several millimeters, it is more difficult to achieve efficient coupling of high power from the waveguide bundle into the much smaller entrance ends of long, thin, low loss, single-mode fiber lasers with multi-mode claddings. Another disadvantage of this arrangement lies in the difficulty one encounters in matching the lateral and transverse numerical apertures of the waveguides with the lateral and transverse divergences of the laser elements.
U.S. Pat. No. 5,127,068 discloses a cylindrical microlens for collimating the high numerical aperture (NA) output emissions of laser diode arrays so that it may be coupled into an optical fiber. The microlens has a diameter roughly equal to the diameter of the fibers and 20% to 50% bigger than the lateral dimension of the laser diode array. The patent mentions that cross-sectional shapes such as elliptical or hyperbolic may prove useful for correction of particular spherical aberrations. Such an arrangement requires a precisely small spacing from the microlens to the optical fibers creating a crowded condition in which unintended contact and damage to the microlens or fibers may occur. Moreover, positional and alignment errors may prevent matching of the numerical apertures of the fibers with the divergences of the laser diodes thus tending to reduce coupling efficiency.
U.S. Pat. No. 5,081,639 discloses a cylindrical microlens which produces focused, defocused, or collimated exiting light and having circular, elliptical, and hyperbolic cross-sectional shapes. Such lenses are well-corrected on axis but rapidly lose correction off-axis. Thus, they have limited practical ability to couple light simply and efficiently from laser diodes into fiber waveguides.
U.S. Pat. No. 4,826,269 discloses focusing diode lasers onto a single region by circularly disposing a number of vertical and horizontal cylindrical lenses. The complexity of such a device makes it unsuitable for many applications.
In as much as there continues to be a need for high power optical fiber lasers, it is a primary object of the present invention to provide such devices.
Another object is to provide an improved coupling system for pumping a solid state medium of a fiber laser cavity.
Other objects of the invention will be apparent, in part, and, in part, will become obvious when reading the following detailed description in conjunction with the drawings.